The present disclosure relates to hydraulic control systems for engines having a variable valve timing mechanism and a valve stop mechanism both of which are operated hydraulically.
Such a hydraulic control system for an engine is disclosed in, e.g., Japanese Unexamined Patent Publication No. 2015-194132.
Engines are provided with many hydraulically operated devices such as a variable valve timing mechanism (VVT) and a valve stop mechanism. Pressurized oil is usually supplied from an oil pump to the hydraulically operated devices via a hydraulic path adjusted to a predetermined oil pressure (target oil pressure).
Cylinder cut-off operation is performed by operation of the valve stop mechanism. In order to maintain the operated state of the valve stop mechanism, an oil pressure (maintaining oil pressure) required to maintain the operated state of the valve stop mechanism need be ensured for the valve stop mechanism. Since fuel efficiency decreases with an increase in target oil pressure, the target oil pressure is recently often adjusted to a minimum required value in order to restrain a decrease in fuel efficiency.
When the VVT is operated during cylinder cut-off operation, oil is supplied to the VVT. This temporarily increases oil consumption and causes reduction in oil pressure, which makes it difficult to ensure the maintaining oil pressure.
The invention of Japanese Unexamined Patent Publication No. 2015-194132 provides a solution to this problem in view of the fact that such reduction in oil pressure increases in proportion to the operating speed of the VVT. Specifically, when the VVT is operated during cylinder cut-off operation, the operating speed of the VVT is limited to ensure the maintaining oil pressure.